IP Supernetting

Supernetting combines two
smaller blocks of contiguous
IP addresses together
into a continuous range of
addresses that form a larger
super-net. You create a
supernet whenever you need
to aggregate blocks of IP
addresses together. The two
most common situation where
you would create and use a
supernet are:

Aggregating several
routes to a contiguous
block of IP addresses,
into a single route
announcment

The term supernetting stems from the idea that at one time there were 'classes'
of IP addresses and that certain classes of addresses were a certain size. By
adding two sets of addresses of one size using supernetting, you created a larger
set of addresses that contained both of the smaller classful
subnets.

Classful addressing identifies a class C addresses as being in the
range of 192.0.0.0 through 223.255.255.255, and each "Class C" contains 256
addresses. Address classes was a system that predates subnet masks, so
technically speaking, there's no such thing as a "Class C mask", but the term
"Class C" is so misused and the concept of classful addresses is so old that the
term "Class C" is synonymous with a block of 256 addresses even when the first
octet is outside the 192-223 range. A single address block such as 192.168.1.0
- 192.168.1.255 has 256 addresses, but the first address is used for the
network and the second is used for the broadcast address. The remaining
254 addresses can be used for computers. If you have more than 254 computers
that need to be on the same network
(to get Windows Domain Browsing working for instance). then you need to create a
superne

You may recall that every IP address contains a host portion that identifies
the individual host and a network portion that identifies which network the
host is attached to. Under the classful addressing scheme, no masks are needed.
The address itself describes which part of the IP address is the network portion
and which part of the address is the host portion.

The idea of supernetting is the idea of borrowing one or more bits from the
network portion of the IP address and using it to create a larger set of hosts.

Supernetting Examples

Supernetting
Example #1:

Expand the following network so that there are twice the number of hosts as
before, and identify the new network and broadcast addresses.

192.168.0.0 / 255.255.255.0

How to Supernet:

1. Ignore everything else
but the mask at first. Use
the network mask to
determine the number of IP
addresses in the current
block:

255.255.255.0 =
11111111.11111111.11111111.00000000

The zeroes on the right
hand side in the mask
indicate how many bits are
used for hosts in the
network. There are eight (8)
zeroes. Eight bits host
means there are 28 = 256 addresses. If we
double 256, we will want 512
addresses in the new range.

2. Adjust the mask to
expand the range of
addresses.

Every bit is twice as large as the bit to the right of it. To double the
addresses, we only need one additional bit.

When we adjusted the
mask, we took away the
rightmost network bit and
added it to the hosts
portion of the network mask.
This is done just by turning
that bit to 'off' (0) in the
mask. This changes the value
of the third octet from 255
to 254 because we took away
the 1's bit from that octet.

3. Calculate the new
network address.

Because the mask "masks
out" the bits in the host
portion, the bits in the
network portion have to be
counted up to figue out the
new address.

In this case, we got
lucky. The starting address
192.168.0.0 remains
unchanged. The first address
in any range of addresses is
always the network address.
The last address in the
range is the network address
with all the host bits
turned on: